it seems the later model ct9's (ct9b, and the hilux vairents) can handle higher boost pressure before they implode like the ct9a/ ct12s do.
in saying thaT though, what i have always found is that anything over 14psi and your power gain to each psi increase drops dramatically.
you start to just push 'hot air' so to speak and not gain allot of extra power. this is when you start to enter the realms of detrimental engine damage that can be caused extreemly easily because the combustion temps rise allot and the combustion mixture becomes way more prone to knock. much the same as a boosted high compression engine with high ignition timing. so you have to be careful on the tune and make sure the air fuel ratios are up to the job in all driving situations.
there are allot of ways to get extra power from the ct9 without running ridiculously high amount of boost.
exhaust manifold - design of this has allot to do with it also.
good brand external wastegate
correct sized exhaust
correct sized intercooler and piping with as shorter route as possible.
good air intake
aftermarket ecu
boost controller
cams
inlet manifold - acis from 5e is a good cheep option.
hybrid turbo
all this should see you to around 180-200bhp reliable on a stock 4e on under 1bar of boost if tuned well
and if your serious....
nos
higher compression